Lecture 6- Gravimetric Analysis (1h)

8/11/2019 Lecture 6- Gravimetric Analysis (1h)

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The Process Separate - by phases

volatilization methods

precipitation methods

selective ppt'n

account for interferencesassure completeness (solubility

equilibrium)



Precipitation:

prec ipitating agent

sample

dissolvedcomponents

Dissolve sample

Add ppt’ing

reagent Filter

Dry

Weigh



Calculating Amount

of Precipitation Reagent

How many mL of 1% potassium chloride would be needed to precipitate all of the

silver in a 0.5 g ore sample that contains 1.5parts per thousand silver? Allow for a 15%excess of the chloride solution.



Gravimetric Overview Measure

tools for gravimetric analysisfilter crucibles, an oven, balance

Calculate stoichiometry

Report values

% composition, ppt, ppm, ppbuncertainty

averagestandard deviation, confidence interval



The Precipitation Process

(from solution)

Particle Size / Filterability

produce particles large enough to be 'caught‘

ideally, produce crystals avoid colloidal suspension

particle size = 1 - 100 nm



Precipitate Formation crystallization

nucleation:particles join to produce aggregates

crystal growthaggregate grows and 'fall out' of solution

We want a few big chunks of precipitate!

supersaturation: more solute than should be present in solutionrelative supersaturation:

a measure of supersaturation, (Q-S)/SQ = actual solute concentration

S = equilibrium solute concentration



Controlling Precipitation Increase S

Increase temperature

Decrease Q Dilute solution

Well mixed (stirring)



pH control of precipitationCa2+ + C2O4

2- CaC2O4 (s)

H2C2O4 2 H+ + C2O42-

Feeder Reaction:



Homogeneous Precipitation Fe3+ + 3 HCO2

- Fe(HCO2 )3nH2O

(NH2 )CO + 3 H2O + heat

HCOOH + OH- + CO2

+ 2 NH4

+

Feeder Reaction:



High Electrolyte Concentration

to Aid Precipitation

Excess charge on colloid creates ionicatmosphere around particle

D.C. Harris, Quantitative Chemical Analysis, 6 th Ed., p686



Composition by

Gravimetric AnalysisNi2+ (aq) + H2DMG Ni(DMG)2 + 2 H+

A 0.8234 g ore sample produced 0.1397 g ofbis(dimethylglyoximate) nickel (II) (FW =288.91 g/mol). Find the nickel content.

Explain how to create a large, filterableprecipitate.



Combustion Analysis

Absorbs

water

Absorbs

CO2

D.C. Harris, Quantitative Chemical Analysis, 6 th Ed., p691

Find the empirical formula for a 13.72 mg organicsample that produced 6.97 mg of water and 28.44mg of carbon dioxide



Gravimetric Interferences

Coprecipitated impurities

Surface adsorption

• Contaminants bound to the surface• Dry the sample

Mixed crystals

• Replacements in the crystal lattice

• Reprecipitation

Occlusions/mechanical entrapment

• Im urit ockets



Gravimetric Overview

Simple

Cheap Glassware

Reagents

ovens, etc.

Balances

Specific Timely

Accurate

Precise

Sensitive

Lecture 6- Gravimetric Analysis (1h)

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